Conduction Blocks - Heart block 1st, 2nd and 3rd

Question 1

First degree heart block definition

Answer 1

Delayed atrio-ventricular conduction through the AV node. Despite this, every atrial impulse leads to a ventricular contraction, meaning every P waves results in a QRS complex. On a ECG this present as a PR interval greater than 0.20 seconds (5 small or 1 big square)

Question 2

Second degree heart block

Answer 2

Some of the atrial impulses do not make it through the AV node to the ventricles. This means that there are instances where p waves do not lead to QRS complexes. There are several patterns of second-degree heart block.

Question 3

Mobitz Type 1 (Wenchebach’s phenomenon)

Answer 3

Atrial impulses become gradually weaker until it does not pass through the AV node. After failing to stimulate a ventricular contraction the atrial impulse returns to being strong. The cycle then repeats.
On an ECG this will show up as an increasing PR interval until the P wave no longer conducts to the ventricles. This culminates in absent QRS complex after a P wave. The PR interval then returns to normal but progressively becomes longer again until another QRS complex is missed. This cycle repeats itself.
PROGESSIVE PR ELONGATION… BEAT DROPPED… NORMAL PR RETURNS `

Question 4

Mobitz Type 2

Answer 4

Intermitted failure or interruption of AV conduction. This results in missing QRS complexes. This is usually a set ratio of P waves to QRS complexes, e.g. 3 P waves to each QRS complex would be referred to as a 3:1 block. The PR interval remains normal. There is a risk of asystole with Mobitz Type 2
PROGESSIVE PR ELONGATION… BEAT DROPPED… PROLONGED PR RETURNS

Question 5

2:1 block

Answer 5

There is where there are 2 P waves for each QRS complex. Every second p wave is not a strong enough atrial impulse to stimulate a QRS complex. It can be caused by Mobitz Type 2 and it is difficult to tell which.

Question 6

Third degree heart block

Answer 6

AV dissociation = Complete heart block - This is no observable relationship between P waves and QRS complexes. Atria + ventricles beat independently of each other. There is a significant risk of asystole with third-degree heart block.
- Ventricular ESCAPE rhythm is sustaining heartbeat
= SAN (BEST) -> is dysfunctional, AVN takes over -> if dysfunctional, vent pacemakers take over (worst, firing rate 20-40bpm)

Question 7

Causes of first degree heart block

Answer 7

Drugs (beta-blocker, CCB, digoxin -> block AVN conduction
Inferior MI

Question 8

Signs and Symptoms of first degree heart block

Answer 8

Asymptomatic

Question 9

Treatment of first degree heart block

Answer 9

No treatment

Question 10

Causes of second degree heart block

Answer 10

Drugs, inferior MI, rheumatic fever

Question 11

Signs and Symptoms of second degree heart block

Answer 11

Syncope, SOB, Chest pain,

Question 12

Treatment of second degree heart block, complete heart block, or previous asystole

Answer 12

Mobitz Type 1 - Does not require pacing unless poorly tolerated
Mobitz Type 2 - Carries high risk of developing complete AV block - should be paced.
FIRST LINE: ATROPINE 500mcg IV
No improvements: Atropine 500mcg IV repeated (up to 6 doses for a total to 3mg)
Other inotropes (such as noradrenaline)
Transcutaneous cardiac pacing (using D.fib)

Question 13

Treatment of second degree heart block, complete heart block, or previous asystole = HIGH RISK OF ASYSTOLE

Answer 13

Temporary transvenous cardiac pacing
Permenant implantable pacemaker when available

Question 14

Causes of complete heart block

Answer 14

Acute MI
HTN
Structural heart disease
Lyme disease

Question 15

Atropine Drug Class

Answer 15

Antimuscarinic medication and works by inhibiting the parasympathetic nervous system. This leads to side effects of pupil dilatation, urinary retention, dry eyes, and constipation